Apparatus and method for detection of vacuum ultraviolet radiation

ABSTRACT

Vacuum ultraviolet radiation detection apparatus ( 10 ) comprises a radiation detector ( 30 ) in a chamber ( 12 ). The detector ( 30 ) receives ultraviolet radiation from a radiation source ( 36 ). The chamber is evacuated using a dry vacuum pump ( 18 ) to a relatively poor vacuum of no less than 5 Pa. UV transparent gas is supplied from a gas supply ( 26 ), into the chamber ( 12 ) at a relatively low flow rate (around 0.1 liters/minute) so as to provide an overall pressure in the chamber ( 12 ) of between 100 and 1,000 Pa. The use of a relatively inexpensive pump coupled with a lower gas flow rate provides significant cost savings.

This application claims benefit of Patent Cooperation Treaty ApplicationNumber PCT/EP2004/009821, filed Sep. 3, 2004, which claims priority fromGreat Britain Patent Application Number 0321070.5, filed Sep. 9, 2003.

TECHNICAL FIELD OF THE INVENTION

This invention relates to ultraviolet spectroscopy. In particular, butnot exclusively, this invention relates to detecting vacuum ultraviolet(VUV) radiation and atomic emission spectra in the VUV region of theelectromagnetic spectrum.

BACKGROUND TO THE INVENTION

Ultraviolet (UV) radiation is electromagnetic radiation lying betweenthe visible and X-ray regions of the spectrum, that is between 380 nmand 5 nm. Vacuum ultraviolet (VUV) radiation is part of the UV region ofthe spectrum in which radiation is absorbed by air. As a result, anyexperiments arranged to detect this VUV radiation are usually performedin a vacuum. The wavelength of VUV radiation is less than 200 nm.

Atomic emission spectroscopy is a well known technique used to determineconstituent molecules or atoms of a sample. When atoms excited to a highenergy state relax to a lower energy state, or to the ground energystate, photons are emitted. The wavelength of the emitted photonscorrelates to the energy gap between the excited state from which theatom relaxed and the relaxation state to which they decay. Differentatomic species have distinct atomic emission spectrum, and so detectionof the spectra can be used to determine constituents of a sample.

The so-called emission lines are generally in the infrared, visible andultraviolet bands of the electromagnetic spectrum. There is particularinterest in detecting atomic emission lines within the vacuumultraviolet region.

Presently, spectral analytical systems using atomic emission radiationsources introduce the sample material to be analysed into an excitationregion. Here it is excited to spectroemissive levels sufficient to emitdetectable radiation which is characteristic of elements in the sample.The resulting emitted radiation is typically disbursed using a gratingor refracting element and analysed spectroscopically to determinequantitatively the elemental composition of a sample. To avoid theabsorption of radiation below 200 nm by air, and to avoid wavelengthshifts associated with changes in the refractive index of gases, thesample material is excited in a gas filled chamber. Variousspectrometers can be accommodated in or on the chamber to detectradiation in different wavebands of the electromagnetic spectrum. Forinstance, a visible waveband spectrometer might be attached to thesample emission chamber so that the spectrometer has a view of theradiation source (excited sample) through an appropriate window.

For detection in the VUV waveband the appropriate spectrometer should bearranged to overcome the absorption and refractive index variationproblems described above. Such problems result in a reduction of thesensitivity of the instrument which limits the detection capability ofthe spectrometer. Also a shift of the spectral lines can result inerroneous measurement of the concentration of elements in the sample.For instance, the problems associated with refractive index might causethe spectral lines to shift across an output detection system such as aCCD array, or an array of photomultipliers, which would requirere-calibration of the spectrometer to correctly assign the detectedspectral lines to their associated elements.

There have been several attempts to mitigate these problems associatedwith detecting optical spectra at wavelengths below 200 nm. In a firstinstance, the optical spectrometer housing or chamber has been evacuatedto high levels of vacuum so as to remove virtually all the gases withinthe spectrometer chamber. An example of such an instrument is theARL4460 spectrometer sold by Thermo Electron Corporation. The highvacuum is typically achieved by using a two-stage pumping system toevacuate the chamber. The pumping system comprises a primary vacuum pumpsuch as a rotary pump or diaphragm pump arranged in series with a highvacuum pump such as a turbo-molecular pump, the turbo-molecular pumpbeing arranged between the evacuated chamber and the primary vacuumpump.

The two stage pump system is necessary to reduce the pressure in thechamber to less than 0.1 Pa (10⁻³ mbar). This level of vacuum isrequired to produce sufficient transparency in the evacuated chamber ofthe optical path. A rotary or diaphragm pump cannot alone achieve therequired pressure levels within the spectrometer housing to provide asufficient level of transparency in the VUV waveband.

This system has several disadvantages, particularly becauseturbo-molecular pumps are relatively expensive and require regularmaintenance. Such a pump cannot operate on its own and must be used inseries with a primary vacuum pump adding to the expense and maintenanceburden. When the pumps are being serviced or maintained the spectrometeris unable to function.

U.S. Pat. No. 4,322,165 and U.S. Pat. No. 5,506,149 describe a secondproposed solution to the problem, wherein the spectrometer iscontinuously purged with a UV-transparent gas so as to expel UVabsorbing gases from the instrument. The gas is required to be of aparticularly high degree of purity, and flow rates through theinstrument are relatively high (typically in the range of 0.5-5 litersper minute). Such gases are relatively expensive because of the highpurity required, and the rate of consumption of the gas makes the costof purging the gas one of the highest consumable costs for a laboratoryusing such spectrometers. This type of system does not, however, requirehigh vacuum pumping systems.

U.S. Pat. No. 5,225,681 attempts to overcome the problems discussedabove by filling a sealed spectrometer housing with UV-transparent gas.A gas pumping system is required to cycle the gas through a gas cleanerso as to remove out-gassed material from components within thespectrometer housing. This gas cleaning system is relatively expensiveand requires regular replacement. If no gas cleaning system is used, theUV-transparent gas becomes progressively contaminated with UV-absorbentgases and the spectrometer performance can be compromised. During itsworking life, the gas cleaning system requires regular maintenanceduring which the spectrometer cannot function.

Furthermore, the system relies on the spectrometer housing being gastight. Any small leaks cause the pressure in the housing to change,which in turn causes the refractive index of the gas within the systemto vary resulting in a shift in wavelength of detected spectral lines.

Furthermore, the absence of a high vacuum pump causes further problemssince, whenever the components within the housing require maintenance,the housing must be breached and the high purity gas is lost as thehousing fills with air. The housing must first be evacuated to a highvacuum before it can be refilled with the high purity transparent gas,which requires a high vacuum pump. Thus, either the system must bepermanently fitted with a high vacuum pump or an appropriate pump mustbe provided and fitted to the system to evacuate the chamber before thespectroanalyser can operate following any maintenance of the system.This refilling process is extremely time consuming, and of course thespectrometer cannot function during this period.

A further system sold by Hilger Analytical Limited (of England) underthe trading name Polyvac® uses a medium-vacuum two-stage rotary vanepump to evacuate the spectrometer housing to approximately 1 Pa (10⁻³mbar). The rotary vane pump uses oil in the pump to achieve the mediumvacuum in the spectrometer housing. Such a pump system is known as a“wet pump”. At the medium vacuum pressure the residual gas componentsare relatively low in concentration and the passage of UV radiation atwavelengths substantially above 140 nm can occur without significantattenuation.

However, oil from the rotary vane pump can enter the system andcontaminate optical components or affect the refractive index of the gaswithin the spectrometer. To reduce this so-called “backstreaming” of therotary vane pump oil into the housing, the spectrometer housing is fedwith a supply of high purity argon gas. The overall pressure in thehousing is typically 12.5 to 25 Pa (0.125 to 0.25 mbar) when the argongas is introduced. Furthermore, the ultimate vacuum produced by a rotaryvane pump is not particularly stable. As the quantities of residualgases in the chamber are at levels where they can still affect thetransmission of wavelengths below 200 nm, variation of the chamberpressure due to the variation in performance of the pump will give riseto varying degrees of absorption, compromising the stability of thespectrometer at UV wavelengths. The supply of the high purity argon gashelps to stabilise the gas pressure within the spectrometer chamber andthereby stablises to some degree, the UV transparency.

The two stage rotary vane pump is relatively expensive. Furthermore,purging of the system using argon only reduces the risk of opticalcontamination by the oil backstreaming from the rotary vane pump; suchoil contamination is by no means eliminated in this system.

SUMMARY OF THE INVENTION

The present invention aims to ameliorate the problems associated withthe prior art and provide a spectrometer or spectroscopy method whichprovides an improvement on the prior art systems.

Accordingly, the present invention in a first aspect provides vacuumultraviolet radiation detection apparatus, comprising; a radiationdetector arranged to receive ultraviolet radiation from a radiationsource, a chamber in which said detector is disposed, a dry vacuum pumparranged to evacuate the chamber to a pressure no lower than about 5 Pa(0.05 mbar), and a gas supply means arranged to supply a substantiallyUV transparent gas into the chamber through a chamber inlet port.

A dry pump has two primary advantages over the Polyvac® system inparticular. Firstly, it does not require oil and so the problem ofbackstreaming is addressed. Secondly, it is relatively less costly thana rotary vane pump or turbomolecular pump.

Nevertheless, currently available dry pumps are only able to produce arelatively poor vacuum. At or above 5 Pascals, the minimum pressure thatthe dry pump can typically achieve, emitted radiation experiencessignificant absorption. As a result, dry pumps (at least without furtherassistance) have previously been considered unusable in VUV spectralanalysis.

The inventors have found, surprisingly, that by addition of asubstantially UV transparent gas to the chamber, absorption of UVradiation below about 200 nm by the residual atmospheric gases (whichthe dry pump is unable to extract) is significantly reduced, such thatthe system operates successfully even with a dry pump.

By “dry pump” is meant a pump which does not require the use of oil orany other liquid to operate.

According to a second aspect of the present invention there is providedvacuum ultraviolet radiation detection apparatus, comprising a radiationdetector arranged to receive ultraviolet radiation from a radiationsource, a chamber in which said detector is disposed, a pump arranged toevacuate the chamber, and a gas supply means arranged to supply asubstantially UV transparent gas into the chamber through a chamberinlet port, at a flow rate which is such as to provide an overallpressure in the chamber of between 100 and 10,000 Pa (1 to 100 mbar).

UV transparent gas is relatively expensive. The present invention thusaddresses the problem of prior art arrangements such as the US patentsidentified in the introduction above, which tend to use large quantitiesof purging gases.

A much lower (at least 5-6 times lower) flow rate of UV transparent gasis employed in the present invention, coupled with a pump to evacuatethe chamber (the above-mentioned prior art tends to operate at or aroundatmospheric pressure). In a preferred embodiment, the pump may be a drypump or the like which further reduces the cost over and above theannual running cost reduction through a decrease in consumables. Whilstthe flow rate of UV transparent gas is, in the present invention,somewhat higher than that of the Polyvac® system described above, itwill be noted that the gas flow in the Polyvac® system is not intendedto operate in combination with a vacuum so as to prevent absorption butis instead essentially to prevent backstreaming of pump oil vapour.

The gas supply means is, in preference, arranged to supply a flow of gasto the chamber at a rate of about 0.1 liters per minute.

The vacuum pump is preferably arranged to remove the UV-transparent gasfrom the housing and the UV-transparent gas is not re-circulated.

Preferably, the apparatus can also comprise radiation detectors whichare operable in other wavebands, such as the near ultraviolet andvisible wavebands, in which case the UV-transparent gas should also besubstantially transparent in these other operational wavebands.

Preferably, the radiation detector comprises a diffraction grating, suchas a holographic diffraction grating, and a CCD array or other detectionsystem arranged to receive diffracted radiation from the grating. Thegrating is arranged so that radiation from the source impinges on thegrating at an angle, to diffract the radiation into its constituentwavelengths.

Whilst the two aspects of the present invention both provide vacuumultraviolet radiation detection apparatus with cheaper componentconsumables than the prior art, it will nevertheless be understood thatthe twin solutions are by no means mutually exclusive and that anapparatus having both a dry pump and a relatively low flow rate of UVtransparent gas will provide particular advantages.

The present invention also provides a method of detecting vacuumultraviolet radiation using a radiation detector disposed in a chamber,said method comprising; evacuating the chamber to a pressure ofsubstantially equal to or greater than 5 Pa (0.05 mbar), supplying asubstantially UV-transparent gas into the chamber, and detecting UVradiation impinging on the radiation detector.

In still a further aspect of the present invention, there is provided amethod of detecting vacuum ultraviolet radiation using a radiationdetector disposed in a chamber, said method comprising evacuating thechamber, supplying a substantially UV transparent gas into the chamberat a rate such as to maintain the pressure in the chamber at between 100Pa and 10,000 Pa (1 and 100 mbar), and detecting UV radiation impingingon the radiation detector.

BRIEF DESCRIPTION OF THE FIGURES

An embodiment of the present invention is now described by way ofexample, and with reference to the accompanying drawing, in which:

FIG. 1 is a schematic diagram of an apparatus embodying the presentinvention.

DESCRIPTION OF AN EMBODIMENT OF THE PRESENT INVENTION

Referring to FIG. 1, apparatus 10 embodying the present invention isshown in highly schematic form. The apparatus comprises a spectrometerhousing or chamber 12 with a window 14 on one wall of the housing. Thehousing comprises an outlet port 16 and a vacuum pump 18 which isarranged to evacuate the housing to low vacuum pressures only. By lowvacuum pressure it is meant a vacuum pressure of between atmosphere and100 Pa (1 mbar). The vacuum pump vents directly to atmosphere via a pumpoutlet 20.

The housing 12 also comprises an inlet port 22. The inlet port isconnected via a valve 24, to a gas supply 26. The gas supply contains aUV transparent gas, such as argon or nitrogen. The valve 24 might be ofa solenoid or needle type or a mass flow controller. Other devicesallowing an appropriate flow adjustment might be used.

A grating 28, or appropriate diffracting/refracting means, is disposedwithin the housing 12 such that radiation passing through the window 14impinges on the grating and is diffracted and dispersed into itsconstituent wavelengths. A detection system 30 is arranged in thehousing such that light diffracted by the grating impinges on thedetector. There may be further optical devices within the housing, suchas a slit 32, lens systems, or the like.

The housing 12 can be attached to a vessel 34 which houses a source 36of electromagnetic radiation. The source can comprise a plasma arc intowhich a sample can be introduced so as to excite the sample to an extentwhere atomic emission lines are emitted from the sample. Ways and meansfor exciting the sample to such a state do not form part of the presentinvention and no further description is required here.

Light emitted from the sample travels along paths indicated by line 40.The light emitted from the source enters the housing 12 through thewindow 14, passes through the slit 32 and impinges on the diffractiongrating 28. Here the light is bent through an angle depending on itswavelength and the diffracting power of the grating 28. From thegrating, the light travels to a detection system 30 where it isdetected. The detector comprises, for example, a CCD array arranged sothat light of a first frequency (or wavelength) impinges at one end ofthe array and light having gradually higher frequencies impinges alongthe length of the array to the second end of the array. The system canbe calibrated so that discrete frequencies or wavelengths of light onlyfall on a certain element of the CCD array, according to certainconditions within the housing, for instance.

The vacuum pump operates to evacuate the housing to a pressure ofbetween 10 and 5000 Pa (0.1 and 50 mbar), as measured with noUV-transparent gas in the housing. In this state, the residual gaswithin the housing comprises components of air that have desorbed fromthe internal surfaces of the housing, the optical components within thehousing and the window 14. In addition, some of the residual gas in thehousing is due to backstreaming of air from atmosphere from pump 18 andinto the housing 12. Typically, this backstreaming air comprises watervapour, oxygen and carbon dioxide which act to absorb ultravioletradiation, to lesser or greater extents.

With these unwanted gases in the housing, the spectrometer performanceis degraded for wavelengths of light below 200 nm. The limit ofdetection of elements whose atomic emission spectral lines fall at orbelow 200 nm is therefore degraded. In some cases this degradation is solarge that determining the concentration of elements within the sampleunder analysis is not possible.

However, by admitting a low flow UV transparent gas into the housing viathe inlet 22 the performance of the spectrometer for wavelengths below200 nm is, surprisingly, greatly improved. It has been found thatsupplying a gas to the housing at a rate which provides an overallpressure within the housing of between about 100 and 10,000 Pa (1 to 100mbar) is sufficient to overcome the atmospheric absorption problems. Thegas flow rate of ultraviolet transparent gas into the housing istypically 0.1 liters per minute to provide an overall pressure in thehousing of some 10 mbar. This flow rate is at least five times lowerthan the purging gas system of the prior art. At present exchange rates,the reduction in flow of high purity gas provides a typical saving ofroughly

2,000 (roughly US$2,250) per year, per spectrometer.

Despite this example of a typical flow rate, it is to be understood thatthe pressure in the chamber is not directly proportional either to thegas flow rate or the pumping rate and that, whilst adjusting the flowrate will clearly alter the pressure, it is the latter that determineswhether or not the atmospheric absorption problems are overcome.

The low vacuum pump 18 is preferably oil free or a so-called “dry pump”.An example of such a pump is a diaphragm vacuum pump. This diaphragmpump operates without the need for oil in the pump parts which areexposed to gases being pumped, or partial vacuum. Thus, any gasesbackstreaming through the pump are free from oil vapour which would, onentering the housing 12, deposit on all internal surfaces, includingoptical components 14, 28 and 30. Progressive oil vapour deposited onthese surfaces reduces their ability to transmit, reflect, diffract,refract or detect light and thereby degrades the spectrometer'sperformance. As a result, using an oil free pump has the effect that aspectrometer embodying the present invention requires less maintenanceto clean optical components within the housing, effectively increasingthe operational lifetime of an instrument embodying the presentinvention.

The continuous flow of ultraviolet transparent gases at relatively lowflow rates into the housing serves to sweep out desorbed gases from theinternal surfaces of the housing and optical components, it also acts aspresenting a relatively high pressure on the vacuum side of the lowvacuum pump 18. Thus, the pump is operating at a pressure above itsultimate operational vacuum pressure. The pump continuously pumps the UVtransparent gas, along with any desorbed or out-gassed material, throughthe pump to atmosphere. This flow of gases through the pump acts toprevent backstreaming of atmospheric gases from the atmosphere side ofthe pump into the spectrometer housing.

The use of a low vacuum pump, and particularly a diaphragm pump,provides a cost saving for the manufacture of spectrometer devicesembodying the invention without any degradation of spectrometerperformance compared to prior art systems. This, coupled with the savingin purge gas costs and low maintenance/high usage capability of aninstrument embodying the present invention, can lead to a significantsaving over the lifetime of the spectrometer.

The valve 24 can be arranged to cooperate with an orifice restrictor sothat the flow rate of gas from container 26 into the housing 12 iscontrolled at a steady rate. Such orifice restrictors can be procuredready calibrated for certain flow rates of certain gases and willcontribute to the stability of the final gas pressure within thespectrometer chamber.

Suitable UV transparent gases include argon or nitrogen, either of whichcan be procured from specialist gas retailers.

The foregoing is a specific description of a preferred embodiment whichis by way of example only and which is not limiting in nature, the scopeof protection being defined in the appendant claims. Variousmodifications can be envisaged by the skilled person. For example, thewindow 14 may consist of or include a lens or some other form of opticalor mechanical device. The spectrometer could be arranged to operate inother wave-bands where atmospheric absorption is problematic.Furthermore, embodiments of the present invention might also be used forother optical experiments outside of the spectroscopy field.

1. Vacuum ultraviolet radiation detection apparatus, comprising; aradiation detector arranged to receive ultraviolet radiation from aradiation source, a chamber in which said detector is disposed, a dryvacuum pump arranged to evacuate the chamber to a pressure substantiallyequal to or greater than 5 Pa (0.05 mbar), and a gas supply meansarranged to supply a substantially UV transparent gas into the chamberthrough a chamber inlet port while the chamber is being evacuated,wherein the overall pressure in the chamber is below atmosphericpressure.
 2. Apparatus according to claim 1, wherein the UV transparentgas is arranged to be supplied to the chamber at a substantiallycontinuous rate.
 3. Apparatus according to claim 1, wherein the inletport comprises a valve arranged to control the flow of the UVtransparent gas.
 4. Apparatus according to claim 1, wherein, duringoperation, the vacuum pump is arranged to remove the UV transparent gasin the chamber to atmosphere at such a rate, and/or the gas supply meansis arranged to supply the gas at such a rate, so as to cause the overallpressure within the chamber to be substantially equal to or greater than100 Pa (1 mbar).
 5. Apparatus according to claim 4, wherein the vacuumpump is arranged to remove the UV transparent gas in the chamber toatmosphere at such a rate, and/or the gas supply means is arranged tosupply the gas at such a rate, so as to cause the overall pressure inthe chamber to be no more than 10,000 Pa (100 mbar).
 6. Apparatusaccording to claim 4, wherein the gas supply means is arranged to causegas flow into the chamber at about 0.1 liters/minute.
 7. Apparatusaccording to claim 1, wherein the vacuum pump is a dry diaphragm vacuumpump.
 8. Apparatus according to claim 1, wherein the UV-transparent gasis argon, nitrogen, or a combination of argon and nitrogen.
 9. Apparatusaccording to claim 1, wherein the radiation detection apparatuscomprises an optical spectrometer.
 10. Apparatus according to claim 9,wherein the spectrometer is arranged to detect vacuum ultravioletradiation and radiation in another waveband.
 11. Apparatus according toclaim 1, wherein the chamber comprises a window arranged to allowradiation to pass therethrough for detection by the radiation detector.12. An apparatus according to claim 1, wherein the gas supply means is anon-recirculating gas supply means.
 13. An apparatus according to claim1, wherein the flow rate of the UV transparent gas and the pumping rateof the pump are each greater than zero but sufficiently low so that anoverall pressure of between 100 and 10,000 Pa (1 to 100 mbar) ismaintained in the chamber.
 14. Vacuum ultraviolet radiation detectionapparatus, comprising: a radiation detector arranged to receiveultraviolet radiation from a radiation source; a chamber in which saiddetector is disposed; a pump arranged to evacuate the chamber; and, agas supply means arranged to supply a substantially UV transparent gasinto the chamber through a chamber inlet port, the flow rate of the UVtransparent gas and the pumping rate of the pump each being greater thanzero but sufficiently low that an overall pressure in the chamber ofbetween 100 and 10,000 Pa (1 to 100 mbar) is maintained.
 15. A method ofdetecting vacuum ultraviolet radiation using a radiation detectordisposed in a chamber, said method comprising; evacuating the chamber toa pressure of substantially equal to or greater than 5 Pa (0.05 mbar),supplying a substantially UV-transparent gas into the chamber while thechamber is being evacuated, such that the overall pressure in thechamber is below atmospheric pressure, and detecting UV radiationimpinging on the radiation detector.
 16. A method according to claim 15,wherein the UV-transparent gas is supplied to the chamber at such arate, and/or the chamber is evacuated at such a rate, that the overallpressure in the chamber is substantially equal to or greater than 100 Pa(1 mbar) during the vacuum pump's operation.
 17. The method of claim 16,wherein the UV transparent gas is supplied to the chamber at such arate, and/or the chamber is evacuated at such a rate, that the overallpressure in the chamber is no more than 10,000 Pa (100 mbar).
 18. Themethod of claim 14, wherein the gas is supplied to the chamber at a rateof about 0.1 liters/minute.
 19. A method of detecting vacuum ultravioletradiation using a radiation detector disposed in a chamber, said methodcomprising: evacuating the chamber; supplying a substantially UVtransparent gas into the chamber while the chamber is being evacuated,the flow rate of the UV transparent gas and the rate of evacuation ofthe chamber each being greater than zero but sufficiently low that thepressure in the chamber is maintained between 100 Pa and 10,000 Pa (1and 100 mbar); and detecting UV radiation impinging on the radiationdetector.
 20. A method according to claim 15, wherein the UV-transparentgas is removed from the chamber by the vacuum pump.